This invention relates to an image formation device such as an electronic copier, more particularly to an image formation device which can control toner concentration accurately. An electronic copier is a device which forms an electrostatic latent image on a charged photoconductor (described hereinafter as a photosensitive drum) in accordance with manuscript information, changes it into a visible image with toner, and transfers and fixes this visible image on transfer paper. Recently, electronic copiers of this type are used in various industrial fields.
FIG. 11 is a block diagram which shows an electronic copier of this type. When the operator pushes a copier button (not shown), the device shown in the drawing starts the copying process. That is, a photosensitive drum 1 which rotates in the direction of the arrow is electrified by an electrode 3 after residual toner on the drum is removed by the blade in a cleaning section 2.
Next, electric charges on the unnecessary portion of the photosensitive drum 1 are removed by the electrostatic removing section 4.
The photosensitive region on the photosensitive drum 1 is exposed by the reflected light from manuscript 6 and an electrostatic latent image is formed on the drum surface. That is, a manuscript 6 irradiated by the light from the exposure section 5 which can move in the direction of the arrow as shown in the drawing, and the reflected light which contains the manuscript information reaches the photosensitive drum 1 via the mirrors M1 to M3 and lens system L, forming an electrostatic latent image on the drum surface by exposing the drum surface. This electrostatic latent image is converted into a visible image through the developing section 7 by absorbing toner. Then, the visible image on the drum surface is transferred to transfer paper (copy paper) at the transfer section 8. The transfer paper onto which the image is transferred is separated from the photosensitive drum 1 and sent to fixing rollers 10 by means of a transfer mechanism 9.
The transfer paper is heated by these fixing rollers 10 and the toner is fixed on the transfer paper, thus ending the copying process.
The section consisting of the mirrors M1, M2, and M3 and the above-mentioned exposure section 5 (the section surrounded by a broken line) constitutes the optical unit 20, which is designed to be movable in the direction of the arrow shown on the drawing by means of a movement mechanism (not shown).
Actually, the first mirror M1 and the exposure section 5 move at twice the speed the second and third mirrors, M2 and M3, in order to maintain the length of the optical path constant. Since light source 21 in the exposure unit 5 consists of, for example, a rod-like fluorescent lamp or halogen lamp which is set perpendicular to the manuscript surface, the optical unit 20 can scan the whole surface of manuscript 6.
The developing unit which is used in an electronic copier of this type employs a two-component developer consisting of the toner and carrier. Durability of this two-component developer deteriorates rapidly when toner concentration reaches a certain value. For example, in the case in which the toner concentrations are at 5% and at 2%, durability in the latter case may be half of the former case. For this reason, it is necessary to control the supply of toner so that toner concentration can be maintained at the desired value.
A toner amount detection system which uses inductance is available to detect toner concentration in a developing unit which employs two-component developer. This system utilizes the fact that the carrier included in the developer is a magnetic substance and detects toner concentration by arranging an inductance sensor which has a coil in the developing agent.
To say it concretely, the fact that the ratio of toner and carrier changes as toner concentration varies and permeability fluctuates is utilized, and toner concentration is obtained, by measuring permeability of the developing agent.
The output voltage of this inductance sensor is compared with the reference voltage, and toner is supplied so that output voltage of the inductance sensor becomes equal to the reference voltage, thus maintaining toner concentration constant.
By the way, output of the above-mentioned inductance sensor fluctuates, and it is necessary to adjust the output by variable resistors, etc. That is, the output voltage of the inductance sensor has to be adjusted to the predetermined value, when the reference concentration of toner is measured.
However, this adjustment may not be accurate. Accordingly, if the output voltage of the inductance sensor is not accurate, the toner concentration is controlled to erroneous values.
Also, when replacing the developing unit, the output voltage of the above-mentioned sensor had to be adjusted for each replaced developing unit, thus causing a lot of trouble.